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This CL makes it so that all tests in layer_tree_host_impl_unittest.cc
pass when the scroll unification flag is turned on. A followup
mechanical CL adds parameterized test suites to ensure it stays that
way.

The main changes here are that under unification, cases where
ScrollBegin would previously cause a scroll to be routed to the main
thread are now handled on the impl thread. In cases where the hit test
was unreliable or had a non-fast scrollable region, the return now
requests a main thread hit test. For simple main thread scrolling
reasons, a hit test isn't necessary.

Also made some minor updates to a few tests to ensure they're running
under the most realistic scenario by making sure they setup the viewport
correctly.

Bug: 1086625
Change-Id: I20fbd4c431c583e0a63241e91a3adaeb298bed0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2222262
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774012}